1. Field of the Invention
The present invention relates to a directional coupler having a main line and a coupled line.
2. Background Art
It is common for a wireless terminal to include a directional coupler to monitor the level of its transmission power. FIG. 36 shows a typical configuration of a directional coupler. Referring to FIG. 36, a main line 500 is a line for transmitting transmission power and is connected between an input port (#1) and an output port (#2). On the other hand, a coupled line 502 is provided to couple out a portion of the transmission power in the main line 500 and is connected between a coupled port (#3) and an isolated port (#4). It should be noted that some directional couplers are formed in a spiral shape to reduce their overall dimensions (see FIG. 37). The performance of a directional coupler is measured by its directivity, which is defined as the ratio of its coupling to isolation. The higher the directivity, the less the influence of the reflected wave from the output port when the power transmitted from the input port to the output port is coupled out to the coupled port. The coupling and isolation of a directional coupler are often frequency dependent, e.g., as shown in FIG. 38 in which the symbol Dir indicates directivity.
A directional coupler is inserted, e.g., between a transmit power amplifier and an antenna, and used, e.g., in a cellular phone unit as shown in FIG. 39. In FIG. 39, the BB-LSI is the core component of the cellular phone unit and exchanges voice and data with an external device and performs signal processing. Further, the RF/IF-IC shown in FIG. 39 is an IC, and receives the transmission signal from the BB-LSI, frequency converts it to a high frequency signal, and supplies the high frequency signal to an amplifier (PA). The RF/IF-IC also receives the received signal from an antenna (ANT), converts it to an intermediate frequency signal, and supplies the intermediate frequency signal to the BB-LSI. The directional coupler is series connected in the transmission line for the transmission signal. The signal appearing on the coupled port of the directional coupler is delivered through a capacitor Cc to a detector DET. This signal is further delivered from the detector to the BB-LSI and provides information for monitoring and controlling the output level of the amplifier.
Thus, since the directional coupler is used to monitor the output level (or output power) of the amplifier, it is desired that the coupled out signal from the coupled port accurately reflect the output level of the amplifier without error. FIG. 40 is a graph showing the relationship between the directivity of the directional coupler and the error in the power measurement by the detector. Generally, a directional coupler must have a directivity of approximately 20 dB or higher to ensure a measurement error of 0.5 dB or less.
For example, Japanese Utility Model Laid-Open Patent Publication No. 02-098534 (1990) discloses a directional coupler with improved directivity. Specifically, this directional coupler includes a wave combiner in which the multiple reflected wave component included in the transmission wave is cancelled out with a wave obtained by phase adjusting the reflected wave, thereby improving the directivity.
However, the configuration disclosed in this patent publication does not permit miniaturization of the directional coupler (i.e., does not allow for a reduction in the circuit size). Another way to improve the directivity of a directional coupler is to make the coupling length between the main line and the coupled line equal to one-quarter wavelength (λ/4) of the operating frequency. However, for example, cellular phone units use 0.8-5 GHz bands. Such low frequencies mean large values of λ/4, making it impossible to reduce the size of the directional coupler if the coupling length between the main line and the coupled line is made equal to λ/4. Further, in the case of directional couplers using a relatively expensive substrate, such as a GaAs substrate, which provides for improved characteristics, there is great need to reduce the size of the couplers in order to reduce the manufacturing cost. This means that even if they use frequency bands higher than the above 0.8-5 GHz bands, it may not be possible to achieve a coupling length of λ/4, resulting in insufficient directivity.